Allen-Bradley 1785-BCM Backup Communication Module

The Allen-Bradley 1785-BCM serves as a specialized communication interface for high-availability PLC-5 systems. This single-slot module manages the synchronization between primary and standby processors. It ensures that the backup controller remains ready to assume control instantly. By using a High-Speed Serial Link (HSSL), the module mirrors critical data tables across processors. This setup prevents production downtime in the event of a primary controller failure. It is a vital component for mission-critical applications requiring 24/7 operational continuity.

Seamless Failover and Fault Detection

This communication module continuously monitors the health of the active PLC-5 processor. It triggers an automatic switchover if it detects power loss or internal hardware faults. Furthermore, the 1785-BCM supports manual transitions for system testing or maintenance. The switchover process occurs at speeds up to 1.2 Megabaud to maintain process integrity. Consequently, the transition remains transparent to the distributed I/O network. An internal 4K-word buffer facilitates smooth data handling during these critical state changes.

Network Compatibility and Installation

The 1785-BCM integrates with Data Highway Plus (DH+) and Remote I/O (RIO) architectures. It resides in a 1771 I/O chassis alongside the processor it supports. While it coordinates the CPU, all I/O modules must reside in distributed remote racks. This hardware separation ensures that a local chassis fault does not compromise the entire network. To ensure secure terminations, technicians must use the 1771-WG wiring arm during installation.

Technical Specifications

• Module Type: Backup Communication Expansion
• Compatibility: PLC-5/15 (Series B), 5/20, 5/25, 5/30, 5/40, 5/60
• Data Link: High-Speed Serial Link (HSSL)
• Maximum Link Speed: 1.2 Megabaud
• Memory Buffer: 4K Words
• Backplane Current: 1.0 Ampere at 5V DC
• Module Weight: 0.56 kg
• Chassis Requirement: 1771 I/O Rack (Single Slot)

Industrial Reliability and Support

Rockwell designed this module for harsh industrial environments with vibration and noise immunity. The front panel includes status indicators for HSSL link health and standby readiness. These LEDs allow maintenance teams to verify redundancy status at a glance. Although the 1785-BCM reached discontinued status in 2011, it remains essential for legacy system support. Using authentic Allen-Bradley hardware preserves the safety ratings of your existing automation infrastructure.